The invention relates in general to erosion resistant coatings and, more particularly, to hard ductile coatings having a layer of titanium nitride, titanium carbide, zirconium nitride or zirconium carbide with precious metal grain boundaries.
Erosion resistant coatings are used to coat a number of tools and parts. Of particular interest is the use of such coatings in the fabrication of compressor hardware for gas turbine engines. Such hardware includes blades and vanes in the compressor of a gas turbine engine.
Efficient compressor operation demands the maintenance of the original compressor hardware geometry. The compressor blades and vanes have airfoil shapes with relatively sharp edges. Gas turbine engines ingest vast quantities of air and this air will inevitably include some amount of particulate material. In the case of aircraft, this may be sand from the runway. Such materials are very abrasive and erosive when they impact compressor hardware at high speeds. The erosion process is detrimental as it reduces the compressor efficiency because of the degradation of the starting airfoil geometry.
Recently, physical and chemical vapor deposited coatings, such as titanium nitride coatings and zirconium nitride coatings have been used to provide a protective layer having good hardness characteristics. U.S. Pat. No. 4,839,245 discloses a zirconium nitride coating in which the average grain size of the zirconium nitride is not greater than about 3000 Angstroms. This grain size of the zirconium nitride is critical in order to produce an effective erosion resistant coating, particularly to prevent damage from high velocity particles hitting a coated article at a 90° angle. While the coating of the '245 patent shows erosion from high velocity particles hitting the coating at 20° and 90°, the coatings are only two-fold more resistant than other coatings known in the art.
U.S. Pat. No. 5,242,753 likewise discloses a zirconium nitride coating wherein the coating has a substoichiometric amount of nitrogen. However, the coating shows erosion and wear resistance over a narrow range of the substoichiometric nitrogen. Optimal erosion resistance is obtained when the nitrogen is at about 41–42 atomic %, with the resistance dropping off sharply on either side of the range.
Finally, U.S. Pat. No. 5,185,211 discloses a titanium nitride coating with non-stoichiometric amounts of nitrogen. As with the coating of the '743 patent, the coating of the '211 patent shows erosion and wear resistance over a narrow range of nitrogen in the titanium nitride. Optimal erosion resistance is obtained when the nitrogen is at about 38–40 atomic %, with the resistance dropping off sharply on either side of the range.
Although the coatings of the prior art have hardness characteristics, they exhibit inherently brittle behavior and their erosion resistance at normal impact decreases markedly with increasing hardness and particle size of the high velocity particulate material. It has been observed that dynamic impact of high velocity particulate materials onto a coated surface of an article can result in micro-deformations, and associated lateral and/or median cracks around the impact site, decreasing the effectiveness of the coating and the useful life span of the coated article.
U.S. Pat. No. 4,591,418 discloses a coating having alternating microscopically thin layers of titanium nitride and a gold alloy. The titanium nitride layers and the gold layers are distinct, unmixed layers. Furthermore, the '418 patent teaches the disadvantages of co-deposition titanium and gold together in the presence of nitrogen because of the formation of gold-titanium intermetallic compounds. Additionally, the coatings of the '418 patent are for manufacturing golden colored articles and, therefore have no need to be erosion resistant to high velocity large particulate materials. In fact, as gold is a ‘soft’ metal and is on the surface of the coating, one would not anticipate that the coating of the '418 patent could be used as an erosion resistant coating.
U.S. Pat. No. 4,680,438 discloses a laminated material for electrical contacts having a surface contact layer of titanium nitride which is then overlaid with a mixed layer of titanium nitride and a precious metal. The composition in the mixed layer is 5–60% titanium and 40–95% of the precious metal. Additionally, the concentration of precious metal in the layer is preferably a gradient, wherein the concentration of the precious metal is low at the junction with the titanium nitride contact layer, increasing to the surface of the coating. There is no teaching in the '438 patent for using the laminated material as an erosion resistant coating to prevent erosion by high velocity particulate material. As gold is a ‘soft’ metal and the amount of gold in the laminated material is high, the laminated material of the '438 patent would not be suitable as an erosion resistant coating.
As can be seen, there is a need for coatings with excellent adhesion and ductility that can protect parts from erosion by high velocity particles and accommodate microdeformation of the component's surface by the impact of large particles without spalling or cracking.